Page 100 - Introduction to Petroleum Engineering

INTERFACIAL TENSION, WETTABILITY, AND CAPILLARY PRESSURE 85
TABLE 5.2 Wetting Condition and Contact Angle
Wetting Condition Contact Angle (°)

Strongly water wet 0–30
Moderately water wet 30–75
Neutrally wet 75–105
Moderately oil wet 105–150
Strongly oil wet 150–180

Figure 5.1a—the water wets the glass. Figure 5.1b shows a droplet of water on a
surface that is oil wet. The oil almost lifts the water off the surface.
Values of contact angle for different wetting conditions are illustrated in Table 5.2
for oil and water in contact with a solid surface. Wettability can be changed by several
factors, including contact with drilling fluids, fluids on the rig floor, and contact of
the core with oxygen or water from the atmosphere.
Capillary pressure can be defined in terms of wetting and nonwetting phases as

p = p nw − p w (5.2)
c
where p is the pressure of the nonwetting phase and p is the pressure of the wetting
w
nw
phase. In the case of a gas–water system, gas is the nonwetting phase and water is the
wetting phase. Capillary pressure in a reservoir is
2σcos θ
p = p − p = (5.3)
c nw w r
where σ is the IFT between wetting and nonwetting phases, r is the pore radius of the
rock, and θ is the contact angle between rock and fluid in a consistent set of units.
Contact angle depends on the wettability of the rock. Equation 5.3 shows that
capillary pressure increases as pore radius decreases.

Example 5.1 Oil–water Capillary Pressure
Capillary pressure of an oil–water system p is 35 psia and water‐phase
cow
pressure p is 2500 psia. Assuming water is the wetting phase, calculate the
w
oil‐phase pressure.
Answer

p = p + p = 2535 psia
o cow w

IFT and wettability are important because they determine the distribution of
phases in porous media, at the small scale of a rock sample, and at reservoir scale.
The distribution of phases at reservoir scale affects reservoir management. The
small‐scale phase distribution determines relative permeabilities, which affect fluid
flow in the reservoir and into the well.